Engine operation involves combustion that generates exhaust gas. During combustion, an air and fuel (air/fuel) mixture is combusted inside a cylinder to drive a piston. The piston rotatably drives a crankshaft that ultimately rotates one or more camshafts. Exhaust gas is created from combustion and is released from the cylinders into an exhaust system. The amount of exhaust gas released is regulated by the opening and/or closing positions of an exhaust valve that is mechanically actuated by a cam lobe coupled to the camshaft. The exhaust gas may contain residuals such as, oxides of nitrogen (NOx) and carbon monoxide (CO).
Retaining exhaust gas inside the cylinder during the exhaust stroke, also known as exhaust gas retention, burns increased levels of NOx during the following combustion stroke and may decrease levels of emissions exiting the engine. Specifically, retaining exhaust gases in the combustion chamber of the cylinder dilutes the air/fuel mixture and slows the burn rate. The reduced burn rate results in increased combustion chamber temperatures for a longer period of time and burns greater amounts of NOx to reduce emissions.
Exhaust gas retention can be accomplished by adjusting the rotational position of the exhaust camshaft to vary the timing of the exhaust valve. The valve timing determines the amount of exhaust that remains in the cylinder during the exhaust stroke. Levels of NOx retained at various speeds and loads are predetermined and programmed in a static reference table.
Although design differences and component wear can effect engine operation, exhaust gas retention is typically limited to the static reference table.